1. Mendelian Genetics
Unit 3 Notes

2. What is genetics?
The passing of traits to the next generation is called inheritance, or heredity

3. Gregor Mendel Gregor Mendel performed cross-pollination in pea plants
Mendel followed various traits in the pea plants he bred
He is known as the “father of genetics”

4. Mendelian Genetics
The parent generation is also known as the P generation
The offspring of this P cross are called the first filial (F1) generation
The second filial (F2) generation is the offspring from the F1 cross

5. Mendel’s Studies Mendel studied seven different traits:
Seed or pea color
Flower color
Seed pod color
Seed shape or texture
Seed pod shape
Stem length
Flower position

6. Genes in Pairs Allele Dominant Recessive
An alternative form of a single gene passed from generation to generation
Dominant
One alleles masks the other
Recessive
The allele that is masked

7. Dominance
An organism with two of the same alleles for a particular trait is homozygous
An organism with two different alleles for a particular trait is heterozygous

8. Genotype and Phenotype
An organism’s allele pairs are called its genotype
The observable characteristic or outward expression of an allele pair is called the phenotype

9. Mendel’s Law of Segregation
Two alleles for each trait separate during meiosis
During fertilization, two alleles for that trait unite
Heterozygous organisms are called hybrids

10. Law of Independent Assortment
Random distribution of alleles occurs during gamete formation
Genes on separate chromosomes sort independently during meiosis
Each allele combination is equally likely to occur

12. Genetic Recombination
The new combination of genes produced by crossing over and independent assortment

13. Twin Studies
Helps scientists separate genetic contributions from environmental contributions
Traits that appear frequently in identical twins are at least partially controlled by heredity
Traits expressed differently in identical twins are strongly influenced by environment

14. Punnett Squares

15. Monohybrid Cross
A cross that involves hybrids for a single trait is called a monohybrid cross

16. Punnett Squares
Used to predict the possible offspring of a cross between two known genotypes

17. Practice!!!

18. Dominance and Multiple Alleles

19. Incomplete Dominance
The heterozygous phenotype is an intermediate phenotype between the two homozygous phenotypes.
It is a mix of physical appearance between the dominant alleles
Dominant alleles represented by a capital letter in crosses
Example: R or W

21. Other ways of writing Incomplete Dominance

22. Codominance
When an individual is heterozygous for such traits, both alleles for one trait in an individual are expressed equally
In other words, 2 types of alleles that are equally dominant
Phenotype of the heterozygote has both traits simultaneously
Both alleles are dominant, so they are both represented by a capital letter: B, W or CB, CW (superscript)

25. Multiple Alleles
Blood groups in humans (this shows both multiple alleles and codominance)
ABO blood groups have three forms of alleles

26. Practice!

27. Sex-Linked Traits

28. Karyotype Studies (not being tested on)
Karyotype—micrograph in which the pairs of homologous chromosomes are arranged in decreasing size
Images of chromosomes stained during metaphase
Chromosomes are arranged in decreasing size to produce a micrograph

30. Telomeres (don’t need to write down)
Telomere caps consist of DNA associated with proteins
Serves a protective function for the structure of the chromosome
They might also be involved in aging and cancer

31. Nondisjunction (not being tested on)
Cell division during which sister chromatids fail to separate properly
If this occurs during Meiosis I or II then the resulting gametes will not have the correct number of chromosomes
Down syndrome: nondisjunction occurred on chromosome 21.
The result is short stature, heart defects, and mental disability

32. Chromosome Inactivation
Barr bodies: the inactivated X chromosome in females
One X chromosome is randomly inactivated in an early embryonic cell
Example: Tortoise shell cats
The gene for coat color is on the X chromosome.
When the cat is heterozygous in coat color (black/orange) it is born with patches of different colored fur on its body.

34. Sex-Linked Traits
Traits are controlled by genes located on the sex chromosomes
Humans: 22 homologous pairs of matching chromosomes called autosomes, and one pair of sex chromosomes (XX or XY)
There are genes found on the X chromosome that are not found on the Y chromosome
Inheritance of traits determined by these genes is therefore linked to the sex of the individual

35. X-linked Traits Genes located on the X chromosome that control traits
Since males have only 1 X chromosome they are affected by recessive X-linked traits
Females are less likely to express a recessive trait because she has two X chromosomes. One of the X chromosomes can mask the affect of the trait.

36. Sex Determination Sex chromosomes determine an individual’s gender
Autosomes are all the other chromosomes that determine everything else. (i.e. Body cells)
The Y chromosome determines the sex of the organism
XX=Female; XY=Male

37. When completing crosses…
When writing genotypes:
Put the trait gene as a superscript to the sex chromosome
XRXR; XRY
Capital letter represents a dominant allele and lowercase letter represents a recessive allele

38. Examples (to name a few)
Red-Green Color Blindness
Hemophilia

39. Ex: Hemophilia
Recessive sex-linked disorder characterized by delayed clotting of the blood.
Very rare in females because she would need to have both X chromosomes with the recessive trait.

40. Types of Genotypes XHXH (female with normal blood clotting)
XHXh (female with normal blood clotting, but is a carrier of the trait)
XhXh (female with hemophilia)
XHY (male with normal blood clotting)
XhY (male with hemophilia)

42. Ex: Red-Green Color Blindness
This is a recessive X-linked trait. A male only needs one copy of this allele in order to be colorblind
A female would need to have two copies of the recessive allele. Thus it is very rare to find a color blind female